A Series Hydraulic Hybrid Drive Train For Off-Road Vehicles
With heavily uctuating uel prices, the total cost o ownership o loaders, excavators, and other o-road machines is nowadays strongly inuenced by the uel costs. Moreover, there is growing concern about CO2emissions caused by the burning o ossil uels as well as about the long-term availability o these uels. The uel economy and efciencies o the drive train and the hydraulic implements have thereore become extremely important parameters in the design o uture o-road machines. Hybrid transmissions are normally not considered to be a solution or o-road machines. Hybrid drive trains are oremost developed or passenger cars where they can beneft rom the recuperation o brake energy. But or many o-road vehicles, brake energy recuperation is not an option. Furthermore, hybrid electric vehicles need sophisticated electric transmissions with delicate and expensive inverters, converters, and batteries. Taking the extreme power transients in mobile machinery and the rough operational conditions o o-road drive trains into account, it is questionable whether the delicate hybrid electric drive trains can be considered a viable, inexpensive, and robust option or o-road applications. Yet, there is still the alternative o the hydraulic hybrid drive train. These are or some years developed or buses, reuse, and delivery trucks [1-6]. This paper describes a hybrid drive train
8 | Off-Highway Directory 2009
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By Peter A.J. Achten, Innas BV Fig. 1: Hybrid electric swing
drive or an excavator [16]
Fig. 2: Measured total efciency o a 28 cc oating cup pump/motor Fig. 3: Low speed torque relative to the maximum theoretical torque Fig 4: Operating points o the diesel engine o a loader [38]
3 1
2
or o-road machines. The drive train –coined the ‘Hydrid’ [7, 8]– is a ull series hybrid system with an in-wheel motor in each wheel and hydraulic transormers or efcient power control. HYBRID ELECTRIC DEVELOPMENTS
There is a common consensus that hybrid drive trains can strongly increase the efciency o a drive train. Especially in the passenger car market segment, the Toyota Prius has proven the viability o the concept. However, the increased efciency does not by itsel result in a reduction o the uel consumption or the cost o ownership. The electric drive train components result in an increased vehicle weight, which increases the uel consumption o the vehicles. Moreover, the added cost or the electric components gives doubt about the cost- beneft-relationship, especially o the ull hybrid drive train concepts [9-15]. Nevertheless, the trend o hybrid electric transmissions has also come to the o-road market. Kobelco and New Holland have
presented an excavator, in which the swing movement o the upper carriage is realized by means o a hybrid electric drive ([16], see Figure 1). Because o the hybrid system, a smaller internal combustion engine could be applied. The uel consumption is reduced by 40%, according to Kobelco. In Europe, Deutz and Atlas Weyhausen have shown a hybrid electric wheel loader, in which the ywheel o the internal combustion engine is replaced by an electric motor/ generator [17]. According to the developers, the uel consumption is reduced by 20%. Finally, also Volvo Construction Equipment (Volvo CE) has announced a concept o a hybrid electric loader called the Gryphin [17, 18]. Unlike the developments o Kobelco and Weyhausen, the mechanical drive train in the Gryphin is completely replaced by an electric drive, having in-wheel electric motors in all our wheels. The hydraulic hybrid system, which is described in this paper, is the hydraulic equivalent
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o the Gryphin, at least or the drive train part o the system. It has an in-wheel hydrostatic machine in each wheel, hydraulic transormers instead o electric power controllers, and hydraulic accumulators instead o batteries. Added to this, the hydraulic hybrid system also includes recuperation modes or the hydraulic cylinders. FLOATING CUP AND HYDRAULIC TRANSFORMERS
A ull hydrostatic drive requires highly efcient pumps
and motors. Current piston pumps and motors have a peak efciency o around 93%. A pump-motor-combination could thereore – in theory – have an efciency o maximum 86%. In reality, the part load operation o the hydrostatic machines reduces the efciency o both the pump and the motor. I, or instance, an average efciency o 80% or both the pump and motor is assumed, the total average efciency o the hydrostatic drive is limited to 64%. The new Floating Cup Principle [19-25] strongly improves the efciency o hydraulic pumps and motors. Recent tests have proven a maximum total efciency o up to 98%. The oating cup principle is a multi piston pump, having 24 pistons. This creates an almost constant torque output. In addition, the oating cup principle has extremely low riction losses at low rotational speeds. The smooth torque and the low riction result in a start-up torque, which is nearly equal to the maximum theoretical output torque o the hydrostatic motor. Figure 3 shows the torque efciency o a oating cup motor compared to two other motor principles, measured at low speed (< 1 rpm). The oating cup principle oers a starting torque, which is about twice as high as the radial piston motor. For the same output torque at start-up, the oating cup motor can thereore be hal the size o a radial piston motor. This is not only important or reducing the size and weight o the hydraulic components, but also or increasing the drive train efciency. I, due to a low starting torque, the motor size needs to be increased, then the same motor will have to run in strong
4
Off-Highway Directory 2009 | 9
part load conditions during normal operation, which oten results in a reduced efciency. The Floating Cup Principle is also applied in the design o hydraulic transormers. A hydraulic transormer is the hydraulic equivalent o a mechanical CVT. Where a CVT converts torque and speed, a hydraulic transormer converts pressure and ow, without principal energy losses. In the hydraulic transormer developed by Innas (the Innas Hydraulic Transormer or IHT [26-37]), the control o the transormer is realized by rotating the port plate. Whereas the port plate in hydrostatic pumps and motors only has two ports, the port plate in the IHT has three ports: one connected to the load, one connected to the highpressure supply side, and one to the low-pressure supply side. There are also hydraulic transormers developed or hydrostatic drive applications, allowing 4-quadrant operation o the wheel motors (i.e. orward propulsion, orward breaking, reverse propulsion, and reverse braking). These transormers have our ports: two or the supply side and two or the load side. Unlike valves, hydraulic transormers are based on a positive displacement principle, and thereore have no principle throttle losses. Being a nondissipative principle, it is also possible to convert hydraulic power to a higher-pressure level. This creates the opportunity or recuperating energy rom any hydraulic load, both wheels and hydraulic cylinders. For instance, in a orklit truck, it is possible to recuperate the break energy o the vehicle as well as the energy rom the lit cylinders while lowering a load at the ork boom [28].
engine can be twice as high as in the optimum or sweet point). In the proposed hydraulic hybrid (or Hydrid) transmission (fgure 5), the engine is supply-
ed to a constant displacement pump. I this would or instance be a 28 cc pump, then the pressure range o the accumulator would result in an engine torque,
Fig. 5:
Hydraulic circuit o the hydraulic hybrid (‘Hydrid’)
HYDRAULIC IMPLEMENTS
6a.
6: Hydraulic circuit or the control o a hydraulic cylinder by means o a hydraulic transormer (6a: rod side always pressurized; Fig.
6b: rod side can be
switched).
6b.
HYDROSTATIC HYBRID DRIVE
In a wheel loader, about hal o the uel consumption is used or the hydraulic cylinders. The other hal goes to the wheels or the propulsion o the vehicle [38, 39]. Figure 4 shows the operating points o the diesel engine o loader or a typical cycle [38]. For a large part, the engine is operated at a load o less then 50% o the maximum load. In this area, the engine efciency strongly deteriorates (at a load o 20% the specifc uel consumption o an 10 | Off-Highway Directory 2009
ing all its power to a so- called common pressure rail (CPR). The pressure at the high- and lowpressure side o the CPR-system is defned by the accumulators. Due to the design o the accumulators, the pressure dierence between the high- and lowpressure side can only be varied in a limited pressure range, or instance between 200 and 350 bar. In the Hydrid, the internal combustion engine is connect-
loads, and a simple grid or energy and power distribution. As with electricity production, more then one power plant can be connected to the Common Pressure Rail. The combined power o all power plants will be chosen as such that it can deliver the maximum power that the vehicle needs or extended periods. This can be lower than the maximum power demanded in a conventional drive train, since the increased efciency also reduces the maximum power demand. In addition, the accumulators can also deliver extra power to the system, thereby acting as peak shaving devices. In most operating points, the power demand o the vehicle is much lower than the maximum power demand. Having two power plants, one o the power plants can be switched o, and the power management can be optimized better or average operating points.
which can only vary between 89 Nm (at 200 bar) and 156 Nm (at 350 bar). As a result, the engine will always be operated at a medium to high load, and the operating points with a low efciency o the engine (and the pump) will be avoided completely. The new driveline is similar to the design o the stationary electricity production and distribution, having a separation between the power plants and
Like the electricity grid, there can be any number o loads connected to the power grid, including hydraulic cylinders. The accumulators will avoid load disturbance reactions rom one load to the other and also allow the recuperation o energy. The hydraulic transormers convert the pressure level oered by the high-pressure rail to the pressure level required by the load, thereby avoiding the throttle losses that would occur with valve control. Figure 6 shows two confgurations or the control o a double-acting hydraulic cylinder by means o a single hydraulic transormer. In the second confguration, the rod side can be switched between the low-pressure rail and the high-pressure side o the CPR-system. In the upper design (fgure 6a), the rod side is always connected to the high-pressure side and the hydraulic transormer must ampliy the pressure at the bottom side o the cylinder in order to compensate or the hydrostatic orce rom the rod side. More details about the control o hydraulic cylinders by means o hydraulic transormers can be ound in the literature [28-37]. Not all hydraulic cylinders need to be controlled by means o a hydraulic transormer. I cylinders are only used occasion-
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By Peter A.J. Achten, Innas BV ally and don’t consume a large amount o energy, a simple and less expensive throttle valve is perectly acceptable or controlling the cylinder. Figure 7 shows two hydraulic diagrams or a orklit truck. The upper diagram is the conventional system with a hydrostatic drive and a loadsensing system or the cylinders. The second diagram shows the CPR- alternative with hydraulic transormers or the hydrostatic wheel motors as well as or the lit cylinder. The other cylinders or tilting and clamping are controlled directly with valves, thereby accepting the (limited) throttle losses. The CPR-system oers a simple layout, and strongly re-
duces the complex design and maintenance o current loadsensing systems. In principle, the CPR-system treats the hydraulic loads (and the power plants) as modules, which can be attached or detached rom the system as long as there is enough power capacity available. ENERGY RECUPERATION
The possibilities or brake energy recuperation in loaders, excavators, and other mobile machineries are oten less than in on-road applications like garbage trucks and city buses. This is or a part due to the act that loaders and other machines don’t drive on asphalt but on sand, mud, and dirt, which re-
7a.
7b. Fig. 7: Comparison o hydraulic circuits or a ork lit truck [28] (7a:
Conventional load sensing and hydrostatic drive system; 7b: Common pressure rail (CPR) system) www.ifps.org | www.fluidpowerjournal.com
Fig. 8: (Normalized) engine power or a wheel loader
during a typical loading cycle [38] sults in a much higher rolling resistance. Moreover, a loader needs the kinetic energy o the vehicle to dig into a pile o dirt. Still, there are many applications in which brake energy recuperation can be o importance. Examples are orklit trucks, dump trucks, and articulated haulers, but also or the swing movement o the upper carriage o an excavator, energy recuperation could be a signifcant contributor to a better uel economy o the vehicle. Depending o the size, the accumulators in the CPR- system can store at least part o the recuperated energy. But, frst and or all, the accumulators are important or allowing power management and load separation. The diagram o Figure 8 shows the variation o the engine power o a loader during a typical loading cycle. The graph is split or the drive power and the power needed or the hydraulic implements. The diagram clearly shows the strong transient power demand o both the drive and the implement unctions o the loader. Hydraulic accumulators are ideal storage devices or power management in these vehicles. They are simple and robust, and they have a high power density, much higher than electric batteries. Also their efciency can be higher than o batteries. In the Hydrid, batteries are however not excluded as an option or storing larger amounts o energy. By combining a hydraulic and an electric machine, energy rom the CPRsystem can be exchanged with electric batteries. Because the hydraulic machine will always operate at a high pressure and load, the efciency o this unit can be high.
Another unction o the accumulators is to avoid load dependency. In a hydraulic system, the oil ow has to be distributed across the various hydraulic loads. Thereby, the oil always ollows the path o least resistance and a load change on one o the hydraulic motors or cylinders can cause a change in the operating speed o all the other motors and cylinders. Load sensing systems compensate or these load disturbance reactions. In the CPR-system, the loads are controlled at the motors and cylinders (oten reerred to as secondary control). The accumulators act as system capacitance separating the individual loads. It is to be expected that the accumulators can play an important role in recuperating energy rom the hydraulic cylinders. Hydraulic cylinders have by defnition a limited stroke and hence a limited amount o energy. On the other hand, the power transients generated by the hydraulic cylinders can be very high, too high to handle or electric batteries but excellent or hydraulic accumulators. The hydraulic transormer is the key enabling technology or dynamically managing the power stream between the accumulators and the hydraulic cylinders, without loss o productivity. IMPROVED FUEL ECONOMY
The hydraulic hybrid improves the uel economy in several ways: • Idle losses of the engine are
avoided. • Losses of the hydrodynamic
torque converter are avoided. • The engine is always oper-
ated around the sweet point. • Throttle losses in the control of
hydraulic cylinders are minimized. Off-Highway Directory 2009 | 1 1
• Energy recuperation can be
maximized.
REFERENCES
• Auxiliaries like steering sys tems and cooling ans can be decoupled rom the engine and optimized rom an efciency point o view. It will depend on the kind o application how much all o these actors will contribute to an improvement o the uel economy. Eriksson [40] has calculated that a load sensing control o a double-acting hydraulic cylinder has an efciency o around 37%. By means o a hydraulic transormer, most o
1. Wendel G.R. (2000) Regenerative hydraulic systems or increased efciency, Proc. int. exposition or power transmission and technical con., NFPA 2. Wu, B. et al (2002) Optimization o power management strategies or a hydraulic hybrid medium truck, Proc. o the 2002 advanced vehicle control con., Hiroshima, Japan, Sept. 2002 3. Drozdz (2005) Hybrid reuse truck easibility study, Transportation Development Centre (TDC) o Transport Canada, Report nr. TP 14431E
For more inormation, contact Peter Achten at:
[email protected]. these losses could be avoided. As or the drive train, the total efciency o a pump, a transormer, and the hydraulic in-wheel motors will not be higher than the current mechanical drive train, but it will certainly improve the average cycle efciency o the engine. The eects on the uel consumption will be similar to the ull hydrostatic drive train, which is developed or a truck application [5, 6]. CONCLUSION
O-road machines are workhorses. Productivity is the most important requirement. Fuel economy however has become equally important. The proposed Hydrid drive and control system enables the design o a new generation o o-road vehicles with a strongly reduced uel consumption, while maintaining (or even improving) the productivity. The Hydrid completely eliminates the mechanical drive train, thereby creating extra degrees o reedom or the suspension and traction control o the vehicle. The hydraulic Common Pressure Rail is a clear power grid to which power plants, loads, and energy storage devices (including electric batteries) can easily be connected, without needing a complete reengineering o the whole system. The oating cup principle and the hydraulic transormers are the key to the proposed system. 12 | Off-Highway Directory 2009
4. Okoye, V.N. et al (2006) Energy recovery and management in pressure coupled hydraulic hybrid bus using new hydraulic transormer and clean diesel combustion technology, Proc. 5.IFK, March 20-22, Aachen Germany 5. Kovach, J. (2007) Hydraulic hybrid vehicle drive technology, http:// parker.mediaroom.com/fle.php/363 /2+Kovach,+Hydraulic+Truck.pd 6. EPA (2006) World’s frst ull hydraulic hybrid in a delivery truck, http://www.epa.gov/otaq/technology/ 42006054.pd 7. Achten, P.A.J. (2007) Changing the paradigm, Proc. o the 10th Scandinavian International Conerence on Fluid Power (SICFP’07), May 21-23, 2007, Tampere Finland 8. Achten, P.A.J. (2007) The Hydrid transmission, SAE 2007-01-4152 9. Graham R. (2001) Comparing the benefts and im- pacts o hybrid electric vehicle options, fnal report, EPRI, USA 10. Alson (2005) Interim report: New powertrain technologies and their projected costs, EPA (USA) report nr. EPA420-R-05-012 11. Heywood, J.B. et al (2004), The perormance o uture ICE and uel cell powered vehicles and their potential eet impact, SAE-paper 2004-01-1011 12. Weiss, M.A. et al (2000) On the road in 2020, MIT (USA). Energy Laboratory Report nr. MIT EL 00-003 13. Smokers R. et al (2006) Review and analysis o the reduction potential and costs o technological and other measures to reduce CO2-
emissions orm passenger cars, fnal report, TNO (the Netherlands) report nr. 06.OR.PT.040.2/RSM 14. Greene D.L. et al (2004) Future potential o hybrid and diesel powertrains in the U.S. light-duty vehicle market, ORNL (USA) report nr.: ORNL/TM-2004/181 15. Rousseau, A, et al (2005), Trade-o between uel economy and cost or advanced vehicle confgurations, 20th Int. Electric Vehicle Symposium (EVS20), Monaco, (April 2-6 2005) 16. Fenoglio, F. (2006) Built around you, http://media.corporateir.net/media_iles/nys/cnh/id/ enoglio.pd 17. Lang, T. (2007) Hydraulik in Baumaschinen, Ölhydraulik + Pneumatik, nr. 7, p. 404-413. 18. O’Sullivan, B. (2007) Hit or Myth?, IVT International, Sept. 2007, p. 62-66 19. Achten, P.A.J. et al (2003) Design and testing o an axial piston pump based on the oating cup principle, Proc. o the 8th Scandinavian International Conerence on Fluid Power, SICFP’03, Tampere, Finland, May 7-9, 2003 20. Achten, P.A.J. (2003) Designing the impossible pump, Proc. Hydraulikdagarna 2003, Linköping, Sweden, June 3-4, 2003 21. Achten, P., van den Brink, T., Schellekens M., Design o a variable displacement oating cup pump, Proc. 9th Scandinavian Int. Con. on Fluid Power, SICFP’05, Linköping, Sweden. 22. Achten, P.A.J., 2005, Volumetric Losses o a Multi Piston Floating Cup Pump, NFPA/IFPE 2005, Las Vegas, March 16-18, 2005 23. Achten, P.A.J., 2004, Power Density o the Floating Cup Axial Piston Principle, IMECE2004-59006, 2004 ASME Int. Mech. Eng. Congress and Exposition, November 13-20, 2004, Anaheim, Caliornia USA 24. Achten, P.A.J., Schellekens, M., Murrenho, H., Deeken, M., 2004, Efciency and Low Speed Behavior o the Floating Cup Pump, SAE 2004-01- 2653 25. Post, W., 2004, Determination o steady-state perormance o Innas Floating Cup type o axial piston pumps (ISO 4409), DCT-report 2004127, Eindhoven Technical University 26. Achten, P.A.J., Fu, Z., Vael, G.E.M. (1997) Transorming uture hydraulics: a new design o a hydraulic transormer, Proc o the Fith Scandinavian Int. Con. on Fluid Power, SICFP ‘97, Linköping, Sweden 27. Achten, P.A.J., Palmberg, J-O. (1999) What a dierence a hole makes
– the commercial value o the Innas Hydraulic Transormer, Proc. SICFP’99, May 26-28, 1999, Tampere, Finland 28. G.E.M. Vael, P.A.J. Achten (1998) The Innas Fork Lit Truck Working under constant pressure, Proc. o 1. IFK, Aachen, Germany, Vol 1, March 17-18, 1998 29. Werndin R. et al (1999) Efciency perormance and control aspects o a hydraulic transormer, Proc. o the Sixth Scandinavian Int. Con. on Fluid Power May 26-28 1999 Tampere, Finland 30. Werndin, R. et al (1999) Efciency perormance and control aspects o a Hydraulic Transormer, Proc. SICFP’99, May 26-28, 1999, Tampere, Finland 31. Werndin, R. (1999) Efciency perormance and control aspects o a Hydraulic Transormer, Thesis Linköping University 32. Werndin, R. (2001) Controller design or a hydraulic transormer, Fith Int. con. on uid power transmission and control (ICFP2001), 3-5 April, 2001 Hangzhou, China 33. Xu Bing et al (2005) The CPR System adopting a new hydraulic transormer to drive loads and its design, Sixth Int. Con. on Fluid Power Transmission and Control (ICFP 2005), Hangzhou, 2005 REFERENCES 34. Vael, G.E.M et al (2000) The Innas Hydraulic Transormer - The key to the hydrostatic common pressure rail, SAE paper 2000- 01-2561 35. Vael, G.E.M. et al (2003) Cylinder Control with the oating cup hydraulic transormer, Proc. SICFP’03 - Tampere Finland 36. Achten, P.A.J., 2002. “Dedicated design o the Hydraulic Transormer”, Proc. IFK.3, Vol. 2, IFAS Aachen, pp. 233-248 37. Van Malsen R. et al (2002) Design o dynamic and efcient hydraulic systems around a simple hydraulic grid, Proc. Int. Exp. or Power Transmission and Techn. Con. and the SAE Int. O-Highway Con- gress, Las Vegas, Nevada, USA, 19-21 March 2002 38. Filla, R. (2005) Dynamic simulation o construction machinery - towards an operator model, Proc. Int. Fluid Power Exhibition (IFPE) 2005 Technical Conerence, pp. 429-438. 39. Filla, R. (2005) Operator and machine models or dynamic simulation o construction machinery, Thesis Linköping University Institute o Technology, Linköping Sweden 40. Eriksson, B. (2006) Study on individual pressure control in energy efcient cylinder drives, Proc. o the 4th FPNI-PhD SymposiumSarasota, June 13- 17, 2006
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